Certain applications depend on exact measurement of individual powder quantities which are intended to be removed from a store and poured in a predetermined quantity into a container, for example. In particular in the medical sphere, powdered medicines are poured into capsules, blister packs or metering devices. In order to exactly provide the active compound quantity required medically, metering tolerances have to be kept as small as possible.
Volumetric metering is in widespread use in order to obtain a high processing speed with simultaneously sufficient accuracy under industrial filling conditions. In addition to roller metering apparatuses or the like, use is made in particular of what are referred to as pipettes which comprise an outer metering casing. The metering casing surrounds a metering chamber in the circumferential direction, wherein the metering chamber has an opening at a free end of the pipette. Such a pipette is plunged with the opening in front into a powder bed. The metering chamber fills with the powder as the pipette is plunged in. The powder quantity predetermined by the volume of the metering chamber remains in the metering chamber when the pipette is drawn out of the powder bed. The powder can be transported by the pipette and poured out of the metering chamber into the designated container.
A differentiation is made between two types of pipette which are used depending on the properties of the powder to be metered. A “compaction pipette” is used in the case of powders having high binding forces. A metering piston is arranged here within the metering casing, the metering piston being axially displaceable relative to the metering casing and by means of which the volume of the metering chamber is initially set. When the compaction pipette is plunged in, the powder in the metering chamber is compacted. Cohesive forces and other binding forces between the powder grains and on the chamber walls automatically keep the powder in the metering chamber. At the final destination, the powder is injected into the target container by means of the metering piston. A problem here is that the powder has to displace the air located in the metering chamber as the powder penetrates the latter. It is possible for an air cushion to arise which results in metering inaccuracies.
In the case of powders having lower binding forces, automatic adhesion in the metering chamber is not ensured. The powder located in the metering chamber may drop out after the pipette is drawn out of the powder bed. In such cases, use is made of “vacuum pipettes”. The metering chamber in this case is delimited by an air-permeable retaining element on the opposite side of its opening in the longitudinal direction. The metering chamber is charged through the retaining element with negative pressure which keeps the powder despite the small binding forces thereof in the metering chamber. The retaining element keeps the powder in the metering chamber counter to the applied negative pressure. At the final destination, instead of the negative pressure, a positive pressure is introduced through the retaining element into the metering chamber as a result of which the powder located there is blown out into the target container.
The air-permeable retaining element is customarily configured as a deep-bed filter consisting of a metal filter cloth with a certain thickness, a certain pore width and a certain mechanical load-bearing capacity. The pore width is matched to the grain size of the powder to be metered such that air, but not powder particles, can pass therethrough. When the vacuum pipette is plunged into the powder bed, the powder presses against the deep-bed filter. The deep-bed filter has to be able to withstand the pressure and also the powder contact pressure produced by the vacuum.
The manufacturing of such vacuum pipettes is cost-intensive since the condition of the deep-bed filters has to be checked after each manufacturing step. The deep-bed filter may be deformed transversely with respect to the plane thereof during the metering process because of the pressures which are in effect, and therefore changes in volume of the metering chamber and therefore metering weight differences occur. The metering accuracy which can be achieved is therefore limited. In addition, clogging of the metal filter cloth reduces the long term stability. If the previous metal filter cloth has been deformed or damaged by excessive forces, refabrication has been unavoidable.